1. Field of the Invention
The invention relates to a method and an apparatus for reducing the nitrogen oxides in flue gas of combustion systems, having a combustion apparatus, heating surfaces following the combustion apparatus and being located in the flue gas stream, and a deNO.sub.x catalyst.
In view of the emissions limits for nitrogen oxides (NO.sub.x), primary and secondary provisions are being taken to reduce the ejection of NO.sub.x. The term primary provisions is understood to mean provisions in which the formation of nitrogen oxides is lessened. Since the formation of nitrogen oxides is highly temperature-dependent, that category includes, among others, all provisions with which the flame temperature is reduced. The term secondary provisions is understood to mean all provisions with which the nitrogen oxide concentration in the flue gas is reduced afterward. To that end, deNO.sub.x catalysts are typically integrated into the flue gas line.
With gas firing, the required limit values for NO.sub.x emissions can be met solely by primary provisions, even without secondary provisions. However, when heating oil is used, even if the usual primary provisions such as injecting water into the flame are employed, the required limit values are not always met. In that case, the use of deNO.sub.x catalysts is necessary. However, the use of deNO.sub.x catalysts entails disadvantages. Those are a pressure loss in the flue gas line, with the attendant worsening of the efficiency of the system, an SO.sub.2 /SO.sub.3 conversion which is especially undesirable because of the attendant ammonium sulfate formation, a gradual deactivation of the catalysts and the attendant need to recharge the catalysts, and a limitation of the startup speed of a gas turbine resulting from a maximum allowable temperature gradient of the catalyst.
Combustion systems such as gas turbines, which are fueled with gas and heating oil in alternation, require the use of deNO.sub.x catalysts in the one case but not in the other. For that reason, such combustion systems are usually equipped with deNO.sub.x catalysts.